The objective of this project is to continue characterization of lactate transport by isolated renal tubules of Garter snake (Thamnophis). Segments of proximal renal tubule will be perfused in vitro with 14C-lactate and 3H-inulin; after perfusion, the tubule will be assayed for cell (lactate). Net lactate transport, fluid absorption and concentration gradients for lactate will be measured, in order to determine the direction of net transport, (secretion or absorption) and the concentration gradient against which lactate transport occurs. The effect of other organic acids such as probenecid or octanoate on lactate transport and fluid absorption will be determined. Simultaneous experiments on lactate uptake by isolated non-perfused tubules will be performed in order to screen many different organic acids and other compounds which might affect lactate transport at the peri-tubular border. Any compounds which are found to either inhibit or stimulate lactate uptake by the non-perfused tubule will be tested for an effect on lactate transport in the perfused tubule as well. Information gained from such studies will provide greater insight into the mechanism of lactate transport by the renal tubule.